The long term objective of this research project is to determine the functional morphology of the entire nervous system of a simple multicellular animal, Hydra. The specific aims are to characterize the morphology and synaptic connections of nerve cells in (i) a nerve plexus in the peduncle, (ii) a nerve ring in the hypostome, and (iii) the gastrodermal nerve net of the gastric region. The project is health related because detailed knowledge of a simple nervous system can provide important insights into the structural basis for behavior patterns. Thus, an ultrastructural analysis of the neuronal architecture in specific regions of Hydra can provide information about the circuitry underlying its primitive behavior patterns. For example, the discovery of sensory cells in the nerve plexus of the peduncle could lead to the identify of morphological substrates for physiological responses to light in this region. The discovery of interneurons without motor functions in the perihypostomal nerve ring could lead to characterization of a primitive type of central nervous system. Finally, the characterization of a primitive gastric nerve net could help explain the development of different cellular morphologies for different functions in evolution of the nervous system. The methodology for achieving these goals consists of a four-pronged approach: (1) transmission electron microscopy (TEM) of serial thin sections of Hydra neurons in situ and partial reconstruction of their morphology and synaptic connections in three dimensions using a computer- generated program: (2) scanning electron microscopy (SEM) of isolated whole neurons with their long thin processes intact for (i) categorizing the types of neurons into a classification scheme, and (ii) comparing morphological features of whole cells with partially reconstructed cells in situ to correlate, where possible, specific locations of recognizable neurons; (3) high voltage electron microscopy (HVEM) of isolated neurons which have been selectively stained with phosphotungstic acid for visualization of dense-cored vesicles in axonal swellings and at synaptic terminals; and (4) immunocytochemistry of neurons and neurosecretory granules in Hydra using the peroxidase-anti- peroxidase and immunogold techinques with antisera to the sequence RFamide. The application of immunocytochemistry to locate a core peptide common to neurosecretory granules and synaptic dense-cored vesicles in neurons of Hydra can provide important information from a comparative point of view on the evolution of neuropeptide cells.